Digital printers produce digitally-represented images typically composed of a plurality of dots printed on a recording medium, such as paper. The quality of the final image depends on the accuracy in the amount of colorant applied and the position in which each dot is printed. It may therefore be affected by malfunctions of the printer's dot-forming elements. With the ever-increasing resolution to be achieved in modern printer technology, such faults may, however, happen quite frequently, due to the large number of dot-forming elements used; replacement parts may not always be readily available and replacement of the malfunctioning dot-forming elements would be rather extensive. Thus, a technique to compensate for malfunctioning dot-forming elements has been adopted according to which the printers are equipped with redundant dot-forming elements and, if a fault is detected in an active dot-forming element, another of the redundant dot-forming elements is used, thereby taking the role of the faulty element. The use of the redundant dot-forming elements is governed by suitable error-hiding print masks, as will be explained below.
Different techniques to print images represented by digital data are known, for example ink-jet (drop-on-demand and continuous-flow), electrophotography (“laser printing”), dye sublimation, and digital photoprint (see, for example, H. Johnson, “Mastering Digital Printing”, Muska & Lipman Publishing, 2003, pp 61-79). In drop-on-demand ink-jet printing droplets of liquid ink are typically directed towards the recording medium. Thermal or piezo-induced pulses cause the droplets of ink to be ejected from the dot-forming elements. The dot-forming elements of an ink-jet printer are sometimes referred to as “nozzles”. For each ink color used, a large number of such nozzles at different positions are normally arranged in one or more print heads. In electrophotographic printing a laser system typically neutralizes a part of a negatively charged surface of a transfer member, normally a rotating drum, conforming with the image to be printed, which is then able to pick up negatively charged toner particles. The toner particles on the drum are then transferred to the recording medium. The dot-forming elements of an electrophotographic printer may be individual laser elements of the laser system (e.g. laser diodes) which neutralize the individual dots on the drum, corresponding to the image dots.
Page-wide-array printers typically have an array of dot-forming elements extending over a width which is at least the full width of the recording medium. The array may be made up of one print head, or may be segmented in several print heads.
In principle, it would be sufficient to equip a printer with only one dot-forming element for each longitudinal raster line of dots to be printed (and, of course, for each ink color); this would form a minimum set of dot-forming elements. However, printers are often equipped with redundant dot-forming elements, i.e. have more than the minimum set, and can use any one of the two or more redundant dot-forming elements to print a certain dot. Alternatively, such a redundancy can also be achieved by a multipass-print operation in which a certain dot may either be printed in the first, second, third, etc. pass. The distribution of the print activity between the redundant dot-forming elements, or between the different passes, is governed by what is called a “print mode”. A print mode typically includes one or more print masks and a definition how these print masks are to be applied. A print mask, in this context, is no physical mask which, for example, would obstruct the path between dot-forming elements and the recording medium, but is rather an array of logic control-data which defines a pattern of image dots that may be printed by a certain set of the redundant dot-forming elements, or in a certain pass. There are several ways of how a print mode may be represented. In a simple representation, a separate print mask is assigned to each set of the redundant dot-forming elements, or each pass, and the cells of the array (corresponding to a certain dot) may only have two values, either logic-0 or logic-1, meaning that the corresponding dot-forming element is inactive, i.e. must not fire, or may be active, i.e. may fire, depending on the actual image content to be printed (examples of such print masks are, for example, described in the co-pending application “Compensation of lateral position changes in printing” by J. L. Molinet et al., filed on Oct. 10, 2003, Ser. No 10/683,784, and assigned to the assignee of the present application). In other representations, the masks for the different sets of the redundant dot-forming elements, or different passes, are combined into one mask, the cell contents of which indicate by which set, or in which pass, a certain dot is to be printed; e.g. a “2” in a certain cell may indicate that the corresponding dot is to be printed by the corresponding nozzle of the second set, or in the second pass (see, for example, EP 0 998 117 A2, para. [0052]). If more than one drop (of the same ink color) may be applied to one and the same dot, each cell of the print mask may contain more than one number; e.g. the first number indicates the set of nozzles, or pass, responsible for applying the first drop, the second number indicates the set or pass responsible for the second drop, etc. (see, for example, EP 0 998 117 A2, para. [0053])
Since ink-jet printers and other types of printers are only capable of a limited number of tonal levels, half toning or dither masks are used to transform input variations in the tonal levels into the form of spatially varying densities. Such masks are usually filled with discriminator values, meaning that a dot-forming element is inactive if the tone level of the respective picture element (pixel) in the input image is below the discriminator value, and that it is active if the tone level is above it. Such halftoning or dither masks may, in principle, be combined with the “redundancy masks” mentioned above.
One motivation in ink-jet printers to use nozzle redundancy and distribute the nozzle activity by means of print masks has been to avoid ink coalescence at adjacent dots which may occur with certain inks and/or recording media. This is achieved by printing only some of the dots by means of the first set of redundant nozzles, or during the first pass, e.g. according to a checkerboard pattern, giving the printed dots some time to dry, and only then printing the remaining dots by the second set of redundant nozzles, or during the second pass, e.g. according to another checkerboard pattern complementary to the first one (see, for example, the co-pending Molinet et al. application). In such a checkerboard pattern, the black fields may e.g. represent a logic-1, and the white fields a logic-0.
Another motivation has been to avoid horizontal pattern or “banding” effects in swath printers having a horizontally reciprocating print head. In such printers, for example, the use of a misdirected or weak nozzle in the swaths in a regular manner could result in a visible banding effect. Distributing the print activity to different nozzles in an irregular (or randomized) manner may reduce such banding effects. For example, EP 0 738 068 A2, U.S. Pat. No. 6,302,511 B1 and EP 0 998 117 A2 disclose the use of irregular, or random, masks to distribute print activity in swath printers to avoid banding effects.
The use of nozzle redundancy and print-activity-distributing print masks, as described above, also enables known nozzle-errors (such as misdirected, weak or inoperative nozzles) to be compensated, or “hidden”. For example, in a printer with two sets of nozzles (i.e. with a redundancy of one), the dots of each raster line may be printed by the first or the second print nozzle associated with this raster line, and, for example, checkerboard print masks will cause the first and second nozzles to be alternately used. If it is known, however, that one of these nozzles, e.g. the second nozzle, is faulty (such faults may, e.g., be detected by an optical analysis of print-outs made, or an acoustic analysis of the drop noise), the other nozzle, here the first nozzle, may also take the role of the second nozzle, i.e. print all the dots of the raster line in question. This may be achieved by modifying the first (e.g. checkerboard-like) print mask such that all fields of the raster line considered here are set to “black” (logic-1), and that the corresponding fields of the second print mask are set to “white” (logic-0) (see, for example, the co-pending Molinet et al. application). As a result, the line may be correctly printed, although one of the nozzles associated with this line is inoperative (incidentally, the fact that ink may now coalesce is acceptable since this is normally less noticeable than missing dots; furthermore, this effect can be reduced if a higher redundancy is available, for example three, four, five, . . . redundant nozzles or passes). It is also known to modify the irregular or randomized masks used to reduce horizontal-banding effects in swath printers to hide faulty nozzles (see, for example, U.S. Pat. No. 6,302,511 B1 and EP 0 998 117 A2).
Another application of print masks has been proposed in U.S. Pat. No. 6,601,935 B2 according to which, when one and the same pattern is to be printed repeatedly, print masks tuned to the pattern evenly distribute the frequency of use of the nozzles so that the lifetime of the print head is prolonged.
The known techniques to hide errors by nozzle redundancy and activity-distributing print masks rely on a prior knowledge of the nozzle status, in order to be able to define the print masks such that they cause the good nozzles to take the role of the faulty ones. Since the nozzle status may change during a print job, but a detection of the nozzle status may take some time, the correct nozzle status is not always known in high-throughput printers during a print job.